In order to recover the H.sub.2 S contained in low concentration, namely a concentration of less than 20% by volume and more particularly between 0.001% and 20% and most particularly ranging from 0.001% to 10% by volume, in gases from various sources, use may especially be made of processes involving a direct catalytic oxidation of H.sub.2 S to sulphur according to the reaction: EQU H.sub.2 S+1/2O.sub.2.fwdarw.S+H.sub.2 O.
In such processes, the H.sub.2 S-containing gas to be treated, mixed with a suitable amount of a gas containing free oxygen, for example air, oxygen or oxygen-enriched air, is brought into contact with a catalyst for oxidizing H.sub.2 S to sulphur by carrying out this contact at temperatures either above the dew point of the sulphur formed, in which case the sulphur formed is present in the vapour state in the reaction medium coming from the reaction, or at temperatures below the dew point of the sulphur formed, in which case the sulphur is deposited on the catalyst, thereby requiring the sulphur-laden catalyst to be periodically regenerated by flushing with a non-oxidizing gas having a temperature of between 200.degree. C. and 500.degree. C.
In particular, the oxidation of H.sub.2 S to sulphur at temperatures above the dew point of the sulphur, that is to say at temperatures greater than approximately 180.degree. C., may be carried out in contact with a catalyst consisting of titanium oxide (EP-A-0,078,690), titanium oxide containing an alkaline-earth metal sulphate (WO-A-8302068), titanium oxide containing nickel oxide and optionally aluminium oxide (EP-A-0,140,045), an oxide of the titanium oxide or zirconium oxide or silica type, combined with one or more compounds of transition metals chosen from Fe, Cu, Zn, Cd, Cr, Mo, W, Co and Ni, preferably Fe, and optionally with one or more compounds of precious metals chosen from Pd, Pt, Ir and Rh, preferably Pd (FR-A-2,511,663) or else a thermally stabilized alumina combined with one or more compounds of transition metals, such as those mentioned above, especially Fe, and optionally with one or more compounds of precious metals chosen from Pd, Pt, Ir and Rh (FR-A-2,540,092).
As regards the oxidation of H.sub.2 S to sulphur at temperatures such that the sulphur formed is deposited on the catalyst, this may be carried out in contact with a catalyst consisting, for example, of one or more compounds such as salts, oxides or sulphides of transition metals, for example, Fe, Cu, Cr, Mo, W, V, Co, Ni, Ag and Mn, in combination with a support of the activated-alumina, bauxite, silica/alumina or zeolite type (FR-A-2,277,877). This oxidation of H.sub.2 S with deposition of sulphur on the catalyst may also be carried out in contact with a catalyst consisting of a catalytic phase, chosen from the oxides, salts or sulphides of the metals V, Mo, W, Ni and Co combined with a support made of active carbon (French Patent Application No. 93/02996 of 16.03.1993).
The catalysts as mentioned above, consisting of a catalytic phase based on at least one oxide, salt or sulphide of a transition metal and combined with a support consisting of at least one material chosen from alumina, titanium oxide, zirconium oxide, silica, zeolites, silica/alumina mixtures, silica/titanium oxide mixtures and active carbon, which are used for the catalytic oxidation of H.sub.2 S to sulphur, still have certain inadequacies in prolonged use. In particular, the catalysts which have an alumina-based support are susceptible to changing over time by sulphurization. With regard to the catalysts whose support consists of active carbon, precautions must be taken during their use to prevent combustion of the support. In addition, for these various catalysts, the catalytic phase impregnating the support has a tendency to migrate into the intersticies of the support, and thereby making it difficult, or indeed often impossible, to recover the metal from the catalytic phase in the spent catalyst. Finally, the aforementioned catalysts have a mediocre thermal conductivity, which means that the temperature within the catalytic beds containing them cannot be effectively controlled by heat exchange with a coolant.
In order to remedy the drawbacks of the catalysts of the abovementioned type, which are used in processes for the catalytic oxidation of H.sub.2 S to sulphur, and thus to obtain a process resulting in an improved sulphur selectivity that continues, lastingly, over time, the Applicant in the citations FR-A-2,727,101 and WO-A-97/19019 has proposed to carry out the direct oxidation of H.sub.2 S to sulphur in contact with a catalyst formed from a silicon carbide support combined with a catalytically active phase containing at least one transition metal, especially Ni, Co, Fe, Cu, Ag, Mn, Mo, Cr, Ti, W and V, in the oxide or salt form and/or in the elemental state.
In particular, the citations FR-A-2,727,101 and WO-A-97/19019 describe an implementation of the process for the direct oxidation of H.sub.2 S to sulphur in contact with the catalyst having a silicon carbide support, in which the oxidation is carried out at temperatures below the dew point of the sulphur formed by this oxidation, which sulphur is deposited on the catalyst, the oxidation catalyst coated with sulphur is then periodically regenerated by flushing with a non-oxidizing gas, this being carried out at temperatures of between 200.degree. C. and 500.degree. C. and preferably between 230.degree. C. and 450.degree. C., in order to vaporize the sulphur retained on the catalyst, and thereafter the regenerated catalyst is cooled down, below the dew point of the sulphur, to the temperature required for the H.sub.2 S oxidation to be carried out again, this cooling being carried out by means of a gas having a temperature of less than 180.degree. C.